>>2846587idk, they just seem to last
yeah, outsole in heel area is getting thin, but all things considered - they still hold well
when buying I just buy leather (even suede like this holds better to abrasion and campfire sparks, than synthetic)
and thicker rubber toe "guard" is a must for me since sometimes I don't lift my feet high enough/scrape them on rocks, so in the past I was ripping toe area too quickly - rubber "guard" solved this
other than that - I just wash them once few months or after getting drenched in mud completely (having dry sand rubbing in crevices for too long isn't good for shoes), apply some DWR and replace insoles every ~2 years